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8.2
Problem Solving in Geometry
with Proportions
What you should learn
GOAL 1 Use properties of
proportions.
GOAL 2 Use proportions to
solve real-life problems,
such as using the scale of a
map in Exs. 41 and 42.
Why you should learn it
RE
USING PROPERTIES OF PROPORTIONS
In Lesson 8.1, you studied the reciprocal property and the cross product property.
Two more properties of proportions, which are especially useful in geometry, are
given below.
You can use the cross product property and the reciprocal property to help prove
these properties in Exercises 36 and 37.E S O F P R O P O RT I O N S
A D D I T I O N A L P R O P E RT I E S O F P R O P O RT I O N S
a
b
c
d
a
c
a
b
c
d
a+b
b
b
d
3. If = , then = .
c+d
d
4. If = , then = .
FE
To solve real-life
problems, such as using a
scale model to calculate the
dimensions of the Titanic in
Example 4.
AL LI
GOAL 1
Using Properties of Proportions
EXAMPLE 1
Tell whether the statement is true.
p
p
r
3
a. If = , then = .
10
5
6
r
c
a
a+3
c+3
b. If = , then = .
4
3
3
4
SOLUTION
p
r
a. = 10
6
Given
p
6
= 10
r
p
3
= 5
r
a
b
MA.B.3.4.1,
1.02, 2.03 MA.C.3.4.1
b
d
The statement is true.
Given
a+3
c+4
= 3
4
Florida
Standards
North Carolina
and
Assessment
Competency
Goals
a
c
Simplify.
c
a
= 4
3
b.
c
d
If }} = }} , then }} = }} .
c+4
4
a
b
c
d
a+b
b
c +d
d
If }} = }} , then }} = }} .
c+3
4
Because ≠ , the conclusions are not equivalent.
The statement is false.
4658.2 Problem Solving in Geometry with Proportions
465
Page 2 of 7
xy
Using
Algebra
Using Properties of Proportions
EXAMPLE 2
AB
BD
AC
CE
Æ
In the diagram = . Find the length of BD.
A
SOLUTION
AB
AC
= BD
CE
INT
NE
ER T
16
30 º 10
= x
10
Substitute.
16
20
= x
10
Simplify.
B
C
10
E
x
D
Cross product property
x=8
HOMEWORK HELP
Visit our Web site
www.mcdougallittell.com
for extra examples.
Given
20x = 160
STUDENT HELP
30
16
Divide each side by 20.
Æ
So, the length of BD is 8.
..........
The geometric mean of two positive numbers a and b is the positive number x
a
x
x
b
b
such that = . If you solve this proportion for x, you find that x = a•,
which is a positive number.
8
12
12
18
For example, the geometric mean of 8 and 18 is 12, because = , and also
because 8•8
1 = 144 = 12.
EXAMPLE 3
RE
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Using a Geometric Mean
PAPER SIZES International
standard paper sizes are commonly
used all over the world. The various sizes
all have the same width-to-length ratios.
Two sizes of paper are shown, called A4
and A3. The distance labeled x is the
geometric mean of 210 mm and 420 mm.
Find the value of x.
STUDENT HELP
210 mm
420 mm
x
x
SOLUTION
Skills Review
For help with
simplifying square
roots, see p. 799.
x
210
= 420
x
x 2 = 210 • 420
466
A3
A4
Write proportion.
Cross product property
x = 210•2
40
Simplify.
x = 2102
• 10•
2
Factor.
x = 210 2
Simplify.
The geometric mean of 210 and 420 is 210 2, or about 297. So, the distance
labeled x in the diagram is about 297 mm.
Chapter 8 Similarity
Page 3 of 7
GOAL 2
USING PROPORTIONS IN REAL LIFE
In general, when solving word problems that involve proportions, there is more
than one correct way to set up the proportion.
EXAMPLE 4
RE
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Solving a Proportion
MODEL BUILDING A scale model of the Titanic is 107.5 inches long
and 11.25 inches wide. The Titanic itself was 882.75 feet long. How
wide was it?
SOLUTION
One way to solve this problem is to set up a proportion that compares the
measurements of the Titanic to the measurements of the scale model.
PROBLEM
SOLVING
STRATEGY
VERBAL
MODEL
LABELS
REASONING
Width of Titanic
Length of Titanic
=
Length of model ship
Width of model ship
Width of Titanic = x
(feet)
Width of model ship = 11.25
(inches)
Length of Titanic = 882.75
(feet)
Length of model ship = 107.5
(inches)
x ft
882.75 ft
= 11.25 in. 107.5 in.
11.25 • (882.75)
x = 107.5
x ≈ 92.4
Substitute.
Multiply each side by 11.25.
Use a calculator.
So, the Titanic was about 92.4 feet wide.
..........
Notice that the proportion in Example 4 contains measurements that are not in
the same units. When writing a proportion with unlike units, the numerators
should have the same units and the denominators should have the same units.
4678.2 Problem Solving in Geometry with Proportions
467
Page 4 of 7
GUIDED PRACTICE
Vocabulary Check
✓
Concept Check
✓
Skill Check
✓
1. If x is the geometric mean of two positive numbers a and b, write a
proportion that relates a, b, and x. }a} = }x}
b
x
y
x
?
x+4
2. If = , then = . y + 5
4
5
5
4
c
b
b
?
3. If = , then = .
2
6
c
?
6
}} ; or 3
2
4. Decide whether the statement is true or false.
r
s
6
15
15
s
true
6
r
If = , then = .
5. Find the geometric mean of 3 and 12. 6
AB
AD
6. In the diagram = .
BC
DE
Substitute the known values into
the proportion and solve for DE. 9
A
2
B
3
6
D
C
7.
UNITED STATES FLAG The official
height-to-width ratio of the United States
flag is 1:1.9. If a United States flag is 6
feet high, how wide is it? 11.4 ft
8.
UNITED STATES FLAG The blue
portion of the United States flag is
called the union. What is the ratio of
the height of the union to the height of
7
the flag? }1}3
E
PRACTICE AND APPLICATIONS
STUDENT HELP
Extra Practice
to help you master
skills is on p. 817.
LOGICAL REASONING Complete the sentence.
2
7
2
? x
9. If = , then = . }y}
x
y
7
?
y
x
x+5
? y + 12
11. If = , then = . }12}
5
12
5
?
y
6
3
x
x
?
10. If = , then = . }34} or }1}7
6
34
y
?
13
x
20
? x+y
12. If = , then = . }y}
7
y
7
?
LOGICAL REASONING Decide whether the statement is true or false.
STUDENT HELP
HOMEWORK HELP
Example 1: Exs. 9–16
Example 2: Exs. 23–28
Example 3: Exs. 17–22,
43
Example 4: Exs. 29–32,
38–42
468
p
p
7
b
7+a
b+2
3
4
13. If = , then = . true 14. If = , then = . false
r
r
a
2
a
2
4
3
c
c
6
m
d+2
12 + m
3+n
3
15. If = , then = . true 16. If = , then = .
6
d+2
10
12
10
12
n
n
true
GEOMETRIC MEAN Find the geometric mean of the two numbers.
17. 3 and 27 9
18. 4 and 16 8
19. 7 and 28 14
20. 2 and 40 45
21. 8 and 20 410
22. 5 and 15 53
Chapter 8 Similarity
Page 5 of 7
PROPERTIES OF PROPORTIONS Use the diagram and the given information
to find the unknown length.
AB
AC
23. GIVEN = , find BD.
BD
CE
VW
VX
11.25 24. GIVEN = , find VX.
WY
XZ
9
A
20
V
16
B
24
C
9
D
6
BT
ES
25. GIVEN = , find TR. 6 }2}
TR
SL
3
E
SP
SQ
26. GIVEN = , find SQ.
SK
SJ
K
2
P
6
S
10
L
J
5
q
S
LJ
MK
6
27. GIVEN = , find JN. 6 }}
7
JN
KP
QU
RV
28. GIVEN = , find SU. 9
QS
RT
q
M
6
12
S
16
J
1
2
17}}
7
R
L
X
3
Z
Y
E
B
4
T
W
K
6
9
N
R
4
T
P
U
V
BLUEPRINTS In Exercises 29
and 30, use the blueprint of the
FOCUS ON
PEOPLE
1
16
house in which inch = 1 foot.
Use a ruler to approximate the
dimension.
29. Find the approximate width of the
house to the nearest 5 feet.
about 25 ft
30. Find the approximate length of the
house to the nearest 5 feet.
31.
RE
of the number of hits to the number of official at-bats. In 1998, Sammy Sosa
of the Chicago Cubs had 643 official at-bats and a batting average of .308.
Use the following verbal model to find the number of hits Sammy Sosa got.
FE
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SAMMY SOSA
was the National
League Most Valuable
Player in 1998. He hit 66
home runs to finish second
to Mark McGwire who hit 70,
a new record.
INT
about 40 ft
BATTING AVERAGE The batting average of a baseball player is the ratio
NE
ER T
APPLICATION LINK
www.mcdougallittell.com
198 hits
32.
Number of hits
Batting average
= Number of at-bats
1.000
CURRENCY EXCHANGE Natalie has relatives in Russia. She decides to
take a trip to Russia to visit them. She took 500 U.S. dollars to the bank to
exchange for Russian rubles. The exchange rate on that day was 22.76 rubles
per U.S. dollar. How many rubles did she get in exchange for the 500 U.S.
dollars? Source: Russia Today 11,380 rubles
8.2 Problem Solving in Geometry with Proportions
469
Page 6 of 7
35. Each side of the equation 33. COORDINATE GEOMETRY The points (º4, º1), (1, 1), and (x, 5) are
represents the slope of
collinear. Find the value of x by solving the proportion below. 11
the line through two of
1 º (º1)
the points; if the points
5º1
= are collinear, the slopes
xº1
1 º (º4)
are the same.
40. Sample answer: Construct 34. COORDINATE GEOMETRY The points (2, 8), (6, 18), and (8, y) are collinear.
Find the value of y by solving the proportion below. 23
a ramp consisting of two
ramps in opposite
y º 18
18 º 8
directions, each 18 ft long.
= 8º6
6
º
2
The first should be 3 ft
high at its beginning
35. CRITICAL THINKING Explain why the method used in Exercises 33 and 34
1
and 1}} ft high at its end,
is a correct way to express that three given points are collinear. See margin.
2
for a rise:run ratio of
36.
PROOF Prove property 3 of proportions (see page 465). See margin.
1
}}. The second would be
12
c
a
a
b
If = , then = .
1
d
b
c
d
1}} ft high at its beginning
2
and ground level at its end.
37.
PROOF Prove property 4 of proportions (see page 465). See margin.
The second ramp also
1
c
a
a+b
c+d
has a rise:run ratio of }}.
If = , then = .
12
d
b
b
d
43. If the two sizes share a
1
dimension, the shorter
RAMP DESIGN Assume that a wheelchair ramp has a slope of ,
12
dimension of A5 paper
which is the maximum slope recommended for a wheelchair ramp.
must be the longer
1
dimension of A6 paper.
38. A wheelchair ramp has a 15 foot run. What is its rise? 1}} ft
4
That is, the length of A6
paper must be 148 mm. Let 39. A wheelchair ramp rises 2 feet. What is its run? 24 ft
x be the width of A6 paper;
148 is the geometric mean 40. You are constructing a wheelchair ramp that must rise 3 feet. Because of
space limitations, you cannot build a continuous ramp with a length greater
than 21 feet. Design a ramp that solves this problem. See margin.
of x and 210. Then
x
148
}} = }} and x ≈ 104 mm.
148
210
FOCUS ON
PEOPLE
HISTORY CONNECTION Part of the Lewis and Clark Trail on which Sacagawea
acted as guide is now known as the Lolo Trail. The map, which shows a
portion of the trail, has a scale of 1 inch = 6.7 miles.
41. Use a ruler to estimate the
RE
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SACAGAWEA
Representing liberty
on the new dollar coin is
Sacagawea, who played a
crucial role in the Lewis and
Clark expedition. She acted
as an interpreter and guide,
and is now given credit for
much of the mission’s
success.
470
E
W
S
Lewis & Clark
Grove
along the trail between
Pheasant
Portable Soup Camp and
Camp
Full Stomach Camp. Then
calculate the actual distance
in miles. about 1}1} in.; about 8 }3} mi
4
Hearty
Meal
Stop
Portable Soup
Camp
Hungery
Creek
42. Estimate the distance
43.
Horse
Sweat
Pass
N
distance (measured in a
straight line) between Lewis
and Clark Grove and
Pheasant Camp. Then
calculate the actual distance
in miles. about }38} in.; about 2}12} mi
Full
Stomach
Camp
Idaho
0
SCALE
8
Writing Size A5 paper has a width of 148 mm and a length of 210 mm.
Size A6, which is the next smaller size, shares a dimension with size A5.
Use the proportional relationship stated in Example 3 and geometric mean to
explain how to determine the length and width of size A6 paper. See margin.
Chapter 8 Similarity
5 mi
Page 7 of 7
Test
Preparation
1
44. MULTIPLE CHOICE There are 24 fish in an aquarium. If of the fish are
8
2
tetras, and of the remaining fish are guppies, how many guppies are in
3
the aquarium? D
A
¡
B
¡
2
C
¡
3
D
¡
10
E
¡
14
16
45. MULTIPLE CHOICE A basketball team had a ratio of wins to losses of 3 :1.
After winning 6 games in a row, the team’s ratio of wins to losses was 5 :1.
How many games had the team won before it won the 6 games in a row? C
★ Challenge
A
¡
B
¡
3
C
¡
6
D
¡
9
E
¡
15
24
46. GOLDEN RECTANGLE A golden rectangle has its length and width in the
1 + 5
golden ratio 2 . If you cut a square away from a golden rectangle, the
shape that remains is also a golden rectangle.
2
1 + 5
2
46. b. } = }} if 2 is the
2
x
geometric mean of
1 + 5 and x. The
geometric mean of
2
x
smaller golden
rectangle
2
1 5
golden rectangle
2
square
a. The diagram indicates that 1 + 5
= 2 + x. Find x. º1 + 5
1 + 5 and º1 + 5 is
(1
+
5)(
º
1+
5) =
b. To prove that the large and small rectangles are both golden rectangles,
2
1 + 5
show that = .
x
2
4 = 2.
EXTRA CHALLENGE
c. Give a decimal approximation for the golden ratio to six decimal places.
1.618034
www.mcdougallittell.com
MIXED REVIEW
FINDING AREA Find the area of the figure described. (Review 1.7)
47. Rectangle: width = 3 m, length = 4 m 12 m2
48. Square: side = 3 cm
9 cm2
49. Triangle: base = 13 cm, height = 4 cm 26 cm2
50. Circle: diameter = 11 ft
about 95 ft2
FINDING ANGLE MEASURES Find the angle measures. (Review 6.5 for 8.3)
m™A = 109°, m™C = 52°
B
C
A
B
B
51.
52.
53. A
128
115
57. A regular pentagon has 5
lines of symmetry (one
from each vertex to the
54.
midpoint of the opposite
side) and rotational
symmetries of 72° and
144°, clockwise and
counterclockwise, about
the center of the pentagon.
57.
A
D
D
71
C
m™C = 115°, m™A = m™D = 65°
D
C
55. A
67
D
80
C
m™A = m™B = 100°, m™C = 80°
B
B
56. A
41
D
C
m™B = 41°, m™C = m™D = 139°
A
B
D
120
C
m™A = m™B = 113°, m™C = 67°
m™A = 90°, m™B = 60°
PENTAGON Describe any symmetry in a regular pentagon ABCDE. (Review 7.2, 7.3)
8.2 Problem Solving in Geometry with Proportions
471